Incomplete atrioventricular block: blocking the transmission of electrical signals from the heart

2021-06-01 03:02 PM

An electrocardiogram has a PR that lasts about 0.3s instead of the usual 0.2s or less. Thus, the first-degree block is defined as a delay in conduction from the atria to the ventricles rather than a complete loss of conduction.

PR interval prolongation - Block-level I

The time interval from the onset of the P wave to the onset of the QRS complex is approximately 0.16s when the heart beats normally. The PR interval decreases with tachycardia and increases with bradycardia. Usually, a PR interval longer than 0.2s is called prolonged PR and the patient is said to have first-degree heart block.

An electrocardiogram has a PR that lasts about 0.3s instead of the usual 0.2s or less. Thus, the first-degree block is defined as a delay in conduction from the atria to the ventricles rather than a complete loss of conduction.

Figure. Long PR interval due to 1st-degree atrioventricular block (lead II).

The PR interval is rarely longer than 0.35-0.45s because, with that interval, conduction through the bundle of His is stalled for so long that conduction stops altogether. Heart diseases such as acute rheumatic fever also increase the PR interval.

Block II

When conduction through the bundle of His (AV bundle) is pricked enough to increase the PR interval to 0.25-0.45s, the action potential is sometimes strong enough to cross the His bundle and ventricles and sometimes not strong enough to transmit by. In that case, there will be P waves but no associated QRS-T complexes, also known as “lost beats ”, of the ventricles. This is called a 2nd degree block.

There are two types of second-degree block. Type I (Wenkebach's cycle) and type 2. Type I is represented by a progressively longer PR interval until there is a loss of ventricular rhythm when this irregular cycle is repeated. It is usually caused by an abnormality of the atrioventricular node. In most cases, this type is present and does not require treatment.

Type II usually has fixed PR intervals but occasionally atrial depolarization but no associated ventricular depolarization. For example, a 2:1 block means that there are 2 lead P waves for 2 QRS complexes, and 1 P wave without the QRS complex. There can be either 3:2 or 3:1 beat. Type II block is usually caused by an abnormality of the His bundle - Purkinje network and requires implantation of a subcutaneous pacemaker to regulate the heart rate.

The image is a PR 0.3 s long and has a lost ventricular beat due to atrial-to-ventricular conduction error.

Figure. Second-degree atrioventricular block, indicating that the ventricles are frequently not receiving excitatory signals (lead V3).